Combine the lemon juice and 1/4 cup of the sugar in a heavy saucepan.,Bring to a boil over high heat and cook until a thick syrup forms, about 10 minutes.,Combine the cream, the remaining 1/2 cup sugar and lemon zest in a heavy saucepan.,Bring to a boil over medium heat.,Meanwhile, Sprinkle the gelatin over 1 cup of the buttermilk in the top of a double boiler.,Let sit until the gelatin softens, about 5 minutes.,Remove the cream mixture from the heat and add the lemon juice mixture.,Add to the gelatin mixture and set over hot water.,Whisk until the gelatin dissolves.,Add the remaining buttermilk and vanilla.,______,Refrigerate until set, at least 4 hours.